1. Field of the Invention
In general, this invention relates to a fuel delivery system for miniature internal combustion engines, more particularly for model aircraft, comprising: a fuel tank which is sealed off against atmosphere and has at least two lines connected to the engine; one line is a fuel outlet line connected to the engine's carburetor and the other line is a pressurization/venting line connected to the engine's exhaust system whereby pressure from the engine's exhaust is the primary motive force supplying fuel to the engine.
More particularly, the exhaust pressure of each engine stroke cycle in combination with the frequency of said cycle also serve as means for metering precise amount of fuel required by the engine for maximum power and consistency.
2. Discussion of Prior Art
Referring to FIG. 1, a typical model airplane engine 3 is a single cylinder engine having a very simple carburetor 4 (of fixed bore single air intake venturi and non-variable fuel jet orifice at a constant throttle valve position, without a fuel bowl) for air and fuel intake and a very simple single-chamber expansion type of muffler 10 connected to the exhaust port for handling and muffling of exhaust gas. The air flow through the air intake venturi produces in the venturi throat a partial vacuum which draws fuel into the intake airstream from the engine fuel tank through the fuel jet. The carburetor has a throttle valve 4a which is adjustable to regulate air and fuel flow to the engine and thereby engine speed. It is well know in the art that venturi partial vacuum is rather weak, resulting in extremely unreliable engine operation due to variation in aircraft attitude in flight causing variation in fuel head pressure and also due to extremely high centrifugal forces imposed on the aircraft during aerobatic maneuvers, routinely exceeding ten times gravitational force.
The most common practice in response to this problem is to supplement carburetor's venturi fuel draw with pressurized fuel utilizing exhaust back-pressure from the muffler 10 via pressure ring nipple 12 placed on the muffler's expansion chamber 10a. Although this method allows more consistent engine operation, it is far from optimum.
Firstly, muffler back-pressure obtained from pressure fitting 12 as illustrated in FIG. 1 is generally not strong enough to prevent excessively lean or rich engine run during more forceful aerobatics, causing the engine to lose power or to quit abruptly thus potentially endangering the airplane and the public. Some engine manufacturers have placed the muffler pressure fitting nipple 12 directly on the muffler's inlet conduit 10a thus closer to the exhaust source, however, this proved to be not an improvement, and can be explained by the fact that exhaust gas velocity is very high (in the range of thousands of feet per second !) in the muffler inlet conduit 10a. Thus, according to Bernoulli's principle of fluids mechanics, this high flow velocity negate any pressure gain as the result of tapping closer to the exhaust source. Making the carburetor venturi smaller hence increasing the fuel draw will cause significant decrease in engine power. Increasing the muffler back-pressure by narrowing muffler tail pipe 14 will have the same effect, while at the same time causing significant increase in engine operating temperature.
Secondly, the conventional fuel delivery system, which is the aforementioned combination of venturi fuel draw and muffler back-pressure, does not produce optimum fuel metering for maximum power and fuel efficiency. (To truly achieve this requires the complexity of the automobile's carburetor, or a closed-loop feedback electronic fuel injection system). It is know in the art that unless a carburetor is of variable-venturi design (most carburetors are not), a doubling in the rate of air intake will cause a four-fold increase in venturi partial vacuum fuel draw. This means that at a constant degree of throttle opening in a model aircraft engine, as the engine increase in revolution per minute (rpm) during acceleration, a disproportionately higher amount of fuel than air will be drawn into the carburetor, causing a rich mixture thus inefficient operation, a decrease in torque and a limit on the engine power potential as it unloads when the engine gains speed as it unloads. Muffler exhaust back-pressure is better in that it varies in direct proportion to the engine's power output. Surprisingly, even if muffler's back-pressure is the sole motive force for fuel delivery, this still does not provide optimal fuel metering, as illustrated in FIG. 2. Referring to FIG. 2, the graph at the top depicts a typical two-stroke cycle glow ignition model airplane engine's power curve (at full throttle opening) as function of the engine's rpm as tested on the dynamometer. (These graphs represent typical engine testing data by miniature engine expert Mike Billington as published in Model Airplane News magazine periodically.) The graph in the middle shows the engine's fuel consumption (at full throttle opening) as function of engine rpm. It is very important to note that even though the engine's horse power (bhp) increases with increase in engine speed, there is a noticeable decrease in fuel consumption at higher engine speed after the torque peak at 11,500 rpm. This is most obvious in curve 1, which represents the engine being tested in open exhaust form, without muffler. This can also be seen in curve 2, representing the engine being tested with stock muffler. As shown in curve 2, peak fuel consumption occurs at 13,000 rpm and falls off noticeably with increase in rpm, while peak bhp occurs later at around 15,000 rpm with a decrease in fuel consumption. Therefore, the engine as tested in the dynamometer must have the needle valve 7 manually re-adjusted to decrease the needle valve's opening for peak power output every time the engine rpm is allowed to increase by reducing load. The carburetors needlevalve 7 is normally not adjustable in a model aircraft while in flight. And yet, the most efficient way to extract power is to have the engine at full throttle turning a large size propeller of low to medium pitch at the rpm of maximum torque while standing still (static rpm) in order to have good acceleration, and as the aircraft attained its maximum airspeed, the in-flight rpm should correspondingly increase to approach the maximum horsepower peak (this is because the propeller's pitch is fixed in a model aircraft). In curve 1, maximum torque (bottom graph) occurs at around 11,500 rpm and maximum horsepower occurs at around 18,000 rpm, or a .about.50% increase in rpm due to engine unloading, for a maximum of 2.15 bhp (1.6 kw). However, actual in-flight engine measurements data by Dave Gierke as published regularly in Model Airplane News magazine for a number of engines have revealed that at full throttle, the in-flight rpm only increases by 5-10% over that of static rpm, using the conventional stock muffler setup as in FIG. 1c. Therefore, if the engine equipped with stock muffler for fuel pressurization is allowed to have a static rpm of 11,500 rpm as is the usual practice, then only 1.3 bhp (0.96 kw) is extracted from the engine while standing still, as shown in curve 2. However, as the engine unloads in flight an expected 10% over static rpm, to 12,650 rpm, the power extracted from the engine is not 1.45 bhp (1.07 kw) as predicted by curve 2, but actually may be equal to or slightly less than 1.3 bhp (0.96 kw) because as the engine unloads in-flight, its fuel-air mixture got richer than necessary for peak power, hence torque decreases and thus a decrease in bhp even with a slight rpm increase.
In real life operation, when the fuel pressure is relatively weak as is in the stock muffler pressure setup in FIG. 1c, the fuel-air mixture must be set richer than required for peak power to compensate for momentary leaning of the mixture in flight due to changing of fuel head pressure when climbing or diving, due to high centrifugal force in a tight turn or loop, or occasional air bubles in the fuel line, etc.. Without this richer-than-peak setting, then a momentary leaning of the mixture will cause a reduction in engine speed, which in turn creates a decrease in fuel delivery and further decrease in engine speed until the engine stops. On the other hand, the richer-than-peak setting will allow the engine speed to increase in response to momentary decrease in fuel supply, and with increase in engine speed comes stronger fuel draw thus allowing continous engine run. The price for a more stable engine run is a further reduction in the amount of power that can be extracted from the engine and poor fuel economy, which is not insignificant when model engine's fuel costs $10 to $15 a gallon. Curve 5 represents the same engine using stock muffler but with a richer-than-peak mixture setting sufficient for a stable engine run throughout aerobatic maneuvers. In the prior art, this problem is partially addressed in U.S. Pat. No. 4,731,992 by Krumscheid, which discloses a mechanism to increase the fuel pressure to the engine in response to a change of the aircraft from horizontal attitude to vertical attitude. Krumscheid's however, is not a complete fuel metering system, and provides no compensation for fuel-air mixture leaning due to centrifugal force or to air bubbles in fuel line or to dirts partially obstructing the fuel orifice in the carburetor. In U.S. Pat. No. 3,967,606 to Perry, a mechanical fuel pump is provided to supply fuel to the engine at a constant pre-set pressure without regard to engine speed by the use of a built-in pressure regulator and a diaphragmatic pump. Thus, Perry's pump can be expected to provide constant fuel supply during drastic changes in flight condition. But since Perry's pump (and all other fuel pump designs for model aircraft engine) cannot respond to variation in engine's fuel demand with varying in engine speed at a constant throttle opening, the pump does not allow for extraction of maximum power potential from the engine, inspite of its inherent considerable degree of complexity and manufacturing cost.
Referring again to FIG. 2, and by comparison of curve 1 (representing maximum power output from a model engine with optimum fuel metering running without muffler back-pressure) to curve 5 (representing actual power extractable from the same engine with stock fuel delivery system, stock muffler back-pressure and with a sufficiently rich mixture for reliability), it is clear that roughly 50% more power can be extracted from a model airplane engine if the following conditions can be met:
the engine must be run with a very free flowing exhaust system that has almost no exhaust back-pressure,
the engine must have a fuel delivery system powerful enough that it is not measurably affected by drastic change in flight condition, even with the use of large-bore carburetor air intake venturi for increase power, and
the fuel metering system must be able to deliver exact amount of fuel required for peak power at all engine speeds and throttle opening by means of adjustable feed-back mechanism.
In real life situation at present time, a 50% increase in power is most commonly obtained by increasing engine displacement by 50%, which is associated with usually tolerable increases in purchasing cost, engine weight, vibration level, and noise level; therefore, the following conditions must also be met:
the improved novel fuel and exhaust system must contribute significantly less increase in cost, in weight, in vibration level and no higher noise level than would an engine of larger displacement of comparable extractable power output.